When capturing low light intensity signals, it may be particularly important to register the light level with the lowest noise possible. Known advanced complementary metal oxide semiconductor (CMOS) image sensors (CIS) make general use of pixels with a so-called four-transistor pinned photodiode (4T-PPD) architecture. In such an architecture, the transistors are used to read out a pinned photodiode (PPD), and some of the transistors can be shared between different pixels. A PPD may help to eliminate or limit several noise sources, for example, PPD reset noise and dark current shot noise, by avoiding interaction of signal electrons with the silicon surface. As such, a PPD may help to significantly to improve signal to noise ratio (SNR). As a result, the remaining dominant noise source shifts to the rest of the readout chain, for example, source follower, column thermal noise, etc.
The design of a PPD generally includes careful optimization of doping concentrations to ensure auto-reset to a predefined pinning voltage at the point that all charges are removed from the diode and thus the diode is said to be empty. Typically, this pinning voltage lies in the range of between around 1.0V and 1.5V, with a circuit voltage, VDD, typically around 3.3V.
SNR can be further improved in two different ways, namely, by further reducing the readout chain noise, and by amplifying the signal itself whilst introducing no or minimal additional noise. Amplification of the signal may be performed as early as possible as all noise sources at intermediate stages are also amplified.
Further, a p-n junction, when reverse biased by a sufficiently large voltage, can break down due to avalanche generation, that is, impact ionization of carriers in the space-charge region. The breakdown happens when the avalanche generation via a positive feedback goes out of control. However, the same avalanche principle can also be utilized as an amplification process when biased just below the breakdown voltage. Generally, the breakdown voltage of a silicon photodiode can be engineered by choosing the appropriate doping concentrations of the p and n side of the junction. By increasing the photodiode doping concentration, the electric field inside the junction also increases and the breakdown voltage, together with the depletion region width, decreases. However, it has been found that, at breakdown voltages below 6V as described by A. I. Biber in 2000 in his thesis entitled “Avalanche Photodiode Image Sensing in Standard Silicon BiCMOS Technology”, the breakdown is primarily caused by tunnelling, rather than impact ionization. In the case where the breakdown is caused by tunnelling, there is no signal amplification.
Unfortunately, the reverse bias voltages that are used tend to be beyond what a typical CIS process are designed for, and the operating voltage range of the photodiode is normally limited by the gate oxide thickness of a CMOS transistor in a chosen technology. Whilst it is possible to increase the pinning voltage in a 4T-PPD architecture so that an avalanche photodiode can be implemented, this tends not to be practical due to changes required in the associated circuitry.
Previous attempts to utilize in-pixel avalanche photodiodes, as described by A. I. Biber mentioned above, and in 2008 by Y. S. Kim et al. in an article entitled “Design and characterization of CMOS avalanche photodiode with charge sensitive preamplifier” and by L. Pancheri et al. in an article entitled “G.F.D Low-Noise Avalanche Photodiode in Standard 0.25 μm CMOS Technology”, required the use of more complex circuitry leading to poor fill factors.
In US-A-2011/0303822, US-A-2012/0292483 and in an article entitled “A charge-multiplication CMOS image sensor suitable for low-light-level imaging” by R. Shimizu et al., 2009, electron-multiplication is described to achieve avalanche amplification. However, electron-multiplication requires a high voltage capability resulting in CMOS process modification. In addition, a worse fill factor is obtained as more gates need to be employed in each pixel.